The present invention relates to the field of semiconductor processing; more specifically, it relates to a method for forming a retrograde ion implant.
Modern semiconductor devices such as N channel field effect transistors (NFETs) and P-channel field effect transistors (PFETs) require careful tailoring of the dopant concentration profile in the channel region of the device in order to control voltage (VT), off currents (IOFF) and short channel effects (SCE). For an NFET, the channel is formed by control of the P-well dopant profile concentration. For a PFET, the channel is formed by control of the N-well dopant profile concentration. Control of the respective N or P-well profile is accomplished by performing at least one low-voltage and low-dose shallow ion implant and at least one high-voltage and high-dose ion retrograde implant, both of the same dopant type. A shallow implant is one in which the implanted species remain relatively close to the silicon surface. A retrograde implant is one in which the highest dopant concentration of the implanted species occurs a distance below the silicon surface. The channel/well profile tailoring ion implant processes may be best understood by reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are partial cross-sectional views illustrating a related art method of forming a P-well or an N-well. In FIG. 1A, formed in a substrate 100 is shallow trench isolation (STI) 105. Formed on a top surface 110 of silicon substrate 100 is a thin oxide layer 115. Formed on a top surface 120 of STI 105 is a photoresist image 125. A low-voltage and low-dose ion implantation of ion species xe2x80x9cX,xe2x80x9d where xe2x80x9cXxe2x80x9d represents boron for a P-well or phosphorus for an N-well, is performed. Ions 130A pass through thin oxide layer 115 and penetrate into substrate 100 forming a shallow portion 135 of well 140. Ions 130B striking photoresist image 125 are absorbed by photoresist image 125. Ions 130C, striking near sidewall 145 of photoresist image 125 are deflected by atoms in the photoresist but image lack sufficient energy to pass through the sidewall of the photoresist image.
In FIG. 1B, a high-voltage and high-dose ion implantation of ion species xe2x80x9cX,xe2x80x9d where xe2x80x9cXxe2x80x9d represents boron or for a P-well or phosphorus for an N-well, is performed. Ions 150A pass through thin oxide layer 115 and penetrate into substrate 100 forming a deep portion 155 of well 135. Ions 150B striking photoresist image 125 are absorbed by the photoresist image. Ions 150C, striking near sidewall 145 of photoresist image 125 penetrate into the photoresist image, are deflected by atoms in photoresist image 125, and have sufficient energy to escape through sidewall 145, pass through thin oxide layer 115 and penetrate into an edge region 160 of well 140. Edge region 160 extends a distance xe2x80x9cWxe2x80x9d into well 140 measured from resist sidewall 145. Edge region 160 extends a depth xe2x80x9cDxe2x80x9d measured from a top surface 165 of thin oxide layer 115. Obviously P-wells or N-wells away from photoresist image 125 are not effected and do not have edge regions, xe2x80x9cDxe2x80x9d can range from about near zero to 0.5 microns and xe2x80x9cWxe2x80x9d can range from about near zero to 1.2 microns. The VT of NFETs and PFETs devices fabricated in wells adjacent to photoresist image 125 can differ from the VT of NFETs and PFETs fabricated in wells away from (non-adjacent) by as much as about 20 to 120 millivolts. The concentration of dopant in the shallow portion 135 of well 140 in edge region 160 can be ten times the concentration of dopant in the rest of shallow portion 135 of well 140.
Since devices fabricated away from edge region 160 or in wells away from a resist sidewall, which will not have an edge region, their VT will not be increased. Integrated circuits fabricated from a mix of edge and non-edge NFETs and PFETs will have some slow devices and some fast devices. Integrated circuits fabricated from a mix of edge and non-edge NFETs and PFETs and will often exhibit asymmetric behavior.
Therefore, what is needed is a method of forming retrograde ion implants that dose not cause increased dopant concentrations in edge regions of P-wells and N-wells.
A first aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming a masking image having a sidewall on the substrate; forming a blocking layer on the substrate and on the masking image; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer.
A second aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming a blocking layer on the substrate; forming a masking image having a sidewall on the blocking layer; and performing a retrograde ion implant through the blocking layer into the substrate, wherein the blocking layer substantially blocks ions scattered at the sidewall of the masking layer.
A third aspect of the present invention is a method of ion implantation comprising: providing a substrate; forming a first blocking layer on the substrate and a second blocking layer on the first blocking layer; forming a masking image having a sidewall on the second blocking layer; and performing a retrograde ion implant through the first and second blocking layer into the substrate, wherein the second or first and second blocking layers substantially blocks ions scattered at the sidewall of the masking layer.